MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) placed in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image on the basis of MR signals generated due to the excitation. The above-described MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
In MRI, EPI (Echo Planar Imaging) is known an as a high speed imaging technique. In EPI, a scan is performed in such a manner that the gradient magnetic field in the readout direction is consecutively inverted at high speed for each nuclear magnetic excitation to cause consecutive echoes (MR signals).
More specifically, in EPI, all the data required for image reconstruction are acquired by generating consecutive gradient echoes while changing phase encode amount in order, after applying an excitation pulse and before the magnetization in the X-Y plane attenuates and disappears because of transverse relaxation.
There are some types of EPI such as EPI of spin echo type which is based on an SE (Spin Echo) technique and acquires a spin echo signal occurring subsequent to an excitation pulse and a refocusing pulse, EPI of an FE (Field Echo) type which is based on an FE technique and acquires an echo signal occurring subsequent to an excitation pulse, and EPI of an FFE (Fast FE) type which is based on a fast FE technique.
In addition, a type of EPI which reconstructs one image by combining data of echo trains obtained by applying plural excitation pulses is referred to as multi-shot EPI, whereas a type of EPI which reconstructs one image by application of one excitation pulse is referred to as SS (single-shot) EPI.
The pulse waveform of the gradient magnetic field in the readout direction in EPI has shorter pulse width and shorter pulse cycle length, as compared with other imaging techniques. In other words, the frequency component of the pulse waveform of the gradient magnetic field in the readout direction in EPI is high, as compared with other imaging techniques.
Meanwhile, a gradient magnetic field pulse is generated by applying a pulse electric current to a gradient coil. A waveform of the pulsed electric current applied to a gradient coil is ideally a block pulse, but actually becomes a trapezoidal wave having a rising edge region and a falling edge region. As a result, a pulse waveform of a gradient magnetic field does not become an ideal block pulse, but becomes a trapezoidal wave having a rising edge region and a falling edge region.
Generally, in high speed imaging techniques such as EPI, pulse width of a gradient magnetic field pulse is short, and a ratio of a rising edge region and a falling edge region in both ends of a pulse to the entire pulse width becomes high. Therefore, it is proposed to use sampled data over the entire pulse width for image reconstruction by sampling data in a rising edge region and a falling edge region as well as sampling data in a flat region of a pulse.
The method of sampling data in a rising edge region and a falling edge region is called Ramp Sampling. The Ramp Sampling gives a shorter data acquisition time, as compared with other methods of sampling data only in a period during which gradient magnetic field intensity is constant.
However, the raw data of MR signals sampled at regular time intervals in a rising edge region and a falling edge region do not become equally-spaced in a k-space, because these MR signals are sampled while the gradient magnetic field in the readout direction is changing. Thus, it is preferable to rearrange the sampled raw data of MR signals before image reconstruction, in such a manner that the sampled raw data become equally-spaced in the k-space. This rearrangement processing is generally called regridding.
In the conventional regridding processing, a waveform of a gradient magnetic field pulse is calculated based on an equivalent circuit model. This equivalent circuit model is close to an actual gradient magnetic field generation system, because skin effect and eddy currents are considered. Then, improvement in accuracy of regridding processing by performing regridding processing based on the waveform of the gradient magnetic field calculated in the equivalent circuit is achieved.
Although the above conventional regridding processing has satisfactory working effects, it is preferable to perform regridding processing as accurately as possible, in order to improve image quality.
Therefore, in MRI, novel technology to perform regridding processing more accurately than conventional technology has been desired.